There have been many devices in use for driving loads such as LEDs, utilizing a dc-dc conversion type power supply circuit adapted to provide an output voltage different from an inputted power supply voltage. A typical load driving device has a power supply circuit that generates a predetermined output voltage and an output current for driving a load, as disclosed in Japanese Patent Application Laid Open No. 2001-313423. For this purpose, the level of the output voltage or the output current supplied to the load is measured to establish a detection voltage or detection current, which is fed back to a control circuit of the power supply circuit.
In such conventional load driving device, the detection voltage is obtained by dividing the output voltage in a voltage dividing circuit having a high resistance. The detection current is obtained by detecting the potential drop across a resistor (referred to as voltage detection resistor) connected in series with the load, whereby the load current flows through the resistor. The detection voltage (or detection current) is compared to a reference value so that the output voltage (current) outputted from the power supply circuit is controlled based on the comparison.
In a portable electronic device such as a cellular phone, the load current is sometimes increased or decreased within a permitted range in response to a request made during service. For example, when the load is a light emitting diode (LED), a request is made to regulate the luminance of the LED to an arbitrary level.
In such a case, the voltage detection resistor connected in series with the load will increase energy loss when the load current is increased. Therefore, the overall efficiency of the electronic device that includes a power supply circuit and a load disadvantageously drops when the load current becomes large (i.e., during a heavy duty).
In another case, a request is made to drive one load with a constant current and at the same time to drive another load with a voltage above a predetermined voltage. In such a case, conventionally it is necessary to provide a further appropriate power supply circuit to meet individual use conditions, which requires additional space and cost for the power supply circuit and load.
It could, therefore, be helpful to provide a load driving device having a dc-dc conversion type power supply circuit that generates an output voltage by converting a power supply voltage (an input voltage), the power supply circuit capable of adjusting the magnitude of the load current within a predetermined range while avoiding the energy loss caused by an increase in the load current, thereby enabling efficient driving of the load.
It could also be helpful to provide a portable apparatus equipped with such a load driving device.
It could further be helpful to provide a load driving device having a dc-dc conversion type power supply circuit that generates an output voltage by converting a power supply voltage, the power supply circuit capable of driving a multiplicity of loads having different use conditions, including at least one constant-current type load and another type of load, and capable of adjusting the magnitude of the load current supplied to the constant-current type load within a predetermined range while maintaining the output voltage to another type of load above a predetermined voltage.
It could still further be helpful to provide a portable electronic apparatus equipped with such a load driving device.